A specific class of climbing gear, known as active protection, is spring loaded so as to provide engagement, even in parallel-sided or slightly flaring cracks. Spring loaded camming devices, SLCD, are the most common type of active protection. Cam members rotate on the end of a load stem. The cams are spring loaded outwards to provide engagement in the crack. A trigger is connected to the cams with small cables to facilitate placement and removal from the crack.
A SLCD is described by U.S. Pat. No. 4,184,657, issued to Raymond Jardine in 1980, and entitled Climbing Aids. This patent teaches a climbing aid comprising a support bar, a spindle mounted on the support bar, at least two cam members pivotally mounted on the spindle and adapted for opposite pivotal movement from a "closed" position to an "open" position, an operating bar slidably mounted on the support bar and connected to each cam member and there being at the opposite end of the support bar to the spindle an attachment point for a climbing rope.
At the time of its invention, the SLCD concept was a great improvement over current technology (chock stones and the like) because it allowed a single piece of protection to be viable over a range of crack sizes. Also, the SLCD enabled secure protection in parallel sided cracks which were previously deemed unprotectable.
Another invention relating to the SLCD is described by U.S. Pat. No. 4,643,377, issued to Tony Christianson in 1987, and entitled Mechanically Expanding Climbing Aid. This patent teaches a climbing aid which includes opposing cam members, two fixed parallel axles on which the opposing cam members pivot separately with crossed radii, an axle joining member, means for attachment of a climbing rope, spring members which act to move the cam members toward their fully expanded positions, and an operating member which is connected to each cam member such that when it is pulled the cam members retract in order to allow insertion or removal of the improved climbing aid into or out of a crack in rock. A distinguishing feature of the device of Christianson is that two axles are provided. The effect of having two axles is that larger cams and larger cam spacing can be utilized (with the same minimum expansion size as in single axle devices), thus providing a larger maximum expansion.
Of great importance to climbers is the range of expansion of a particular piece climbing aid, where range of expansion is the distance between minimum and maximum cam expansion widths. In order to compare the present invention to prior art devices the term expansion ratio (ER) is now defined as:
ER=maximum expansion width/minimum expansion width
The single axle type has a lower ER than the double axle type. The minimum and maximum expansion of a single axle SLCD are determined wholly by the size of the cams. The average ER of commercial single axle SLCD is approximately 1.55.
The minimum and maximum expansions of a double axle SLCD are determined by cam size and the spacing of the axles. The average ER of commercial double fixed axle SLCD is approximately 1.67. A need exists for a climbing aid with a greater ER.